The present invention relates to a silicon oxide film formed on the surfaces of a plastic substrate. More specifically, the invention relates to a silicon oxide film which has excellent gas shut-off property and is useful in a field of packing materials.
As packing containers, there have heretofore been used metal cans, glass bottles and a variety of plastic containers. Among them, plastic containers have such advantages that they are light in weight and are excellent in shock resistance to some extent accompanied, however, by such problems as permitting the contents to be degenerated and flavor to be decreased due to oxygen that permeates through the container walls.
In metal cans and glass bottles, in particular, no oxygen permeates through the container wall, and what causes a problem is only the oxygen remaining in the containers. In the case of plastic containers, on the other hand, oxygen permeates through the container walls to a degree that is no longer negligible arousing a problem from the standpoint of preserving the contents.
In order to prevent this, there has been proposed a plastic container having a container wall of a multi-layer structure at least one of the layers being formed of an oxygen-blocking resin such as an ethylene/vinyl alcohol copolymer.
However, a multi-layer plastic container requires a technology such as co-extrusion or co-injection of a plurality of resins, a cumbersome formation operation as compared to forming a single-layer resin container, accompanied by a problem of low productivity.
It has also been known already to improve the gas shut-off property by forming a film by vapor deposition on a plastic material of a single layer, and to form a silicon oxide film (SiOx) as well as to form a hard carbon film (DLC).
Japanese Unexamined Utility Model Publication (Kokai) No. 50563/1974 and Japanese Unexamined Patent Publication (Kokai) No. 58171/1974 are teaching silicon oxide films by coating a plastic film based on a physical vaporization method (PVD).
Further, Japanese Unexamined Patent Publication (Kokai) No. 345383/1993 teaches a silicon oxide film formed by the chemical vaporization method (CVD).
Japanese Patent No. 2526766 filed by the present applicant discloses a gas-blocking laminated plastic material comprising a plastic member, a first layer of a polymer formed thereon and containing not less than 15% of silicon, not less than 20% of carbon and the remainder of oxygen, and a second layer of a silicon oxide film formed on the first layer.
However, the conventional silicon oxide film must have a considerably large thickness to impart the required gas shut-off property. Besides, the coated film lacks adhesion to the plastic substrate, softness and flexibility. When, for example, the plastic substrate coated with the above film is drawn, the film is subject to be broken. The productivity is poor, either.
In particular, the silicon oxide film formed by the physical vaporization method (PVD) has inferior oxygen gas shut-off property as compared on the basis of the same film thickness. To achieve the gas shut-off property of the same level, therefore, the film must be formed maintaining a considerably large thickness.
It is therefore an object of the present invention to provide a silicon oxide film having particularly excellent gas shut-off property (gas barrier property), capable of excellently shutting off gases with a small film thickness as compared to the conventional films, the film that is deposited exhibiting excellent adhesion to the plastic substrate, softness and flexibility, lending itself well for being excellently produced.
According to the present invention, there is provided a silicon oxide film formed on the surfaces of a plastic substrate, wherein methyl groups and methylene groups are contained in the silicon oxide film in a portion near the interface to the plastic substrate.
The fact that the methyl groups and the methylenes group are existing in the silicon oxide film of the invention in a portion close to the interface to the plastic substrate, can be confirmed by, for example, depositing an Al film on the surface of the silicon oxide film formed on the surface of the plastic substrate, eluting the plastic substrate by using an organic solvent such as hexafluoroisopropanol or the like, and measuring a first infrared absorption spectrum of the surface of the remaining silicon oxide film. That is, in the first infrared absorption spectrum, an infrared absorption peak due to the methyl group and an infrared absorption peak due to the methylene group appear in a region of wave numbers of from 2800 to 3000 cmxe2x88x921. These peaks make it possible to confirm the presence of the methyl groups and methylene groups. Further, the first infrared absorption spectrum contains an infrared absorption peak due to SiO in a region of wave numbers of from 1000 to 1300 cmxe2x88x921 and, particularly, near 1200 cmxe2x88x921.
By using a secondary ion mass analyzer (SIMS), further, distributions of SiCH2 ions and SiCH3 ions in the film from the outer surface of the silicon oxide film toward the surface of the substrate are measured to make sure the positions where the SiCH2 ions and SiCH3 ions due to an organosilicon compound polymer are present, from which it is obvious that they are not existing on the outer surface of the film but are existing near the interface to the plastic substrate.
That is, in the silicon oxide film of the present invention, the organic groups (methyl groups and methylene groups) are existing near the interface to the plastic substrate accounting for excellent adhesion to the plastic substrate and flexibility. Even when the plastic substrate is intensely drawn, therefore, the film is effectively prevented from being broken.
In the present invention, further, the silicon oxide film has a two-layer structure comprising a first layer positioned on the side of the interface to the plastic substrate and a second layer on the first layer (i.e., layer positioned on the front surface side of the film). The methyl groups and the methylene groups are more distributed in the first layer than in the second layer. It is desired that the methyl groups and the methylene groups are not substantially contained in the second layer.
In the second infrared absorption spectrum of the silicon oxide film of the invention, for example, it is desired that an absorption peak exists in a region of wave numbers of from 1215 to 1250 cmxe2x88x921. The second infrared absorption spectrum is measured by the multiplex reflection differential spectral method from the film surface (surface of the second layer). The above first infrared absorption spectrum chiefly represents infrared absorption characteristics of the first layer positioned near the interface to the plastic substrate while the second infrared absorption spectrum chiefly represents infrared absorption characteristics of the second layer.
That is, owing to the above-mentioned two-layer structure, the silicon oxide film of the present invention exhibits excellent gas shut-off property.
In the second infrared absorption spectrum of the silicon oxide film, further, it is desired that the absorbency ratio defined by the following formula (1),
Ri=A1/A2xc3x97100xe2x80x83xe2x80x83(1) 
wherein A1 is an area of an absorbency of wave numbers over a range of from 1215 to 1250 cmxe2x88x921, and
A2 is an area of an absorbency of wave numbers over a range of from 985 to 1250 cmxe2x88x921, is not smaller than 1%.
In the second infrared absorption spectrum of the silicon oxide film of the present invention, further, it is desired that the infrared absorbency ratio of SiOH/SiO is not larger than 0.25.
It is desired that the silicon oxide film of the present invention has a silicon distribution coefficient represented by a ratio of the silicon content and the film thickness (silicon content/thickness) of 0.3 g/cm3, has an oxygen permeation coefficient of not larger than 0.5xc3x9710xe2x88x9216 ccxc2x7cm/cm2/sec/cmHg (30xc2x0 C.) and, further, has a 10-point average surface roughness (Rz) of smaller than 25 nm and a center line average roughness (Ra) of smaller than 10 nm.
The silicon oxide film of the present invention having the above properties can be produced by a plasma CVD method and, usually, has a thickness of as very small as from 2 to 500 nm yet exhibiting excellent gas shut-off property.
According to the present invention, further, there is provided a gas-blocking plastic material having an inner layer and an outer layer of a thermoplastic resin, and an oxygen-absorbing layer between the inner layer and the outer layer, wherein the above silicon oxide film is formed on the surface of the inner layer and/or on the surface of the outer layer.
That is, upon forming the silicon oxide film on the inner surface and/or on the outer surface of the plastic substrate having the oxygen-absorbing layer as described above, there are imparted oxygen shut-off effect due to the oxygen-absorbing layer as well as gas shut-off property due to the silicon oxide film, making it possible to strikingly improve the gas shut-off property.